This invention involves a process for measuring the flow as a function of time of the several layers of a subterranean multilayer hydrocarbon-producing formation through which a well is drilled, and of the formation.
Measurements of pressure in oil wells as a function of time in order to determine the characteristics of the productive subterranean formations through which the wells are drilled, have long been known. Although such measurements make it possible to determine a considerable number of parameters characterizing subterranean formations in general, they are insufficient in the case of complex reservoirs such as multilayer formations. A single pressure curve cannot in effect supply the data necessary for the determining the characteristics specific to the various layers, such as their permeability and skin coefficient.
A process for testing multilayer systems was proposed by Gao ("The Crossflow Behavior and the Determination of Reservoir Parameters by Drawdown Tests in Multilayer Reservoirs", SPE paper No. 12580, submitted for publication Sept. 29, 1983). Using the semipermeable wall model published by Deans and Gao in SPE paper No. 11966 presented at the 58th Annual Conference and Exposition at San Francisco, Oct. 5-8, 1983, this process consists of testing each layer individually and recording a series of pressure curves. Such a process involves at least three inconveniences. First, it takes a long time. Second, the interpretation of the curves is tricky if there is any transfer flow between formation layers. Finally, during testing, the well is never in an activity mode similar to a real production situation.
Another method of investigating multilayer systems is to use variations of flow and pressure as a function of depth in a stabilized well, i.e., a well in which production is at a constant surface pressure and flowrate. This type of measurement leads to a "snapshot" of the flow and pressure at each layer for a given surface flowrate and pressure. The data obtained can be presented for various successive surface flowrates in the form of a series of pressure/flow curves for each layer. Here, there are two inconveniences. First, not all wells reach a stabilized flow situation. In addition, it was shown (Lefkovits, H. C., Hzebroek, P., Allen, E. E. and Matthews, C. S.: "A Study of the Behavior of Bounded Reservoirs Composed of Stratified Layers", J. Pet. Tech., March 1961) that the respective flowrates of the layers vary with time. Thus, this process is applicable only to wells which actually reach a steady state.